/Engine/Plugins/2D/Paper2D/Source/Paper2D/Classes/PaperSprite.h
Evaluates the SourceUV/SourceDimensons rectangle, finding the tightest bounds that still include all pixels with alpha above AlphaThreshold. The output box position is the top left corner of the box, not the center.
void FindTextureBoundingBox ( float AlphaThreshold, FVector2D & OutBoxPosition, FVector2D & OutBoxSize )